System components which are critical for the system as a whole need reliable lubrication. Often, this is the case for bearings. It is known to employ automated lubrication systems including one or more grease pumps for this purpose. The choice of appropriate lubricants is crucial for the functioning of automated lubrication systems as it has to be made sure that the lubricant reaches its intended destination. Even if a satisfactory lubricant has been identified, ecological and economical demands as well as new developments in the field of tribology and rheology may require changing the type of lubricant employed.
However, many types of grease do not lend themselves to being pumped. The property “pumpability” is understood to refer to the behavior of lubricants when they are being pumped under the operating conditions to be anticipated.
Among the different types of lubricant, the rheological behavior of grease is particularly complex because grease is a multiphase product including base oils and one or more filler materials. Under certain loads and within its range of temperature application, grease exhibits the properties of a solid body, undergoes plastic strain and starts to flow like a fluid should the load reach the critical point, and regains solid-body properties after the removal of the stress.
One of the parameters frequently measured or indicated is the grease consistency classified according to a scale developed by the NLGI (National Lubricating Grease Institute). The scale is based on the degree of penetration achieved by allowing a standard cone to sink into the grease, which has been worked for 60 strokes in a grease worker, at a temperature of 25° C. for a period of 5 seconds. The depth of penetration is measured on a scale of 10-1 mm. The softer greases allow the cone to penetrate further into the grease, hence have a higher the penetration number. The test method is in accordance to ISO 2137.
Unfortunately, it is virtually impossible to determine the grease consistency using the ISO 2137 method under field conditions. The applicants have therefore developed grease test kit with a simple consistency test, wherein a grease sample with a predetermined volume and an initially circular shape is sandwiched between two circular glass plates and then pressed for 15 seconds by putting a weight on the stack. The grease spot expands due to the weight and the resulting diameter is measured and used as an indicator for the approximate NLGI number.
However, the grease consistency is not sufficient to determine the pumpability because different greases with the same consistency may have different pumpabilities.
The pumpability of grease is nowadays mainly determined in a laboratory using so-called ventmeters, wherein the grease is pressurized in a long tube and then released by opening a venting valve. The remaining pressure of the grease in the tube is used as an indicator for the pumpability and for recommendations for a minimum tube diameter required for using the tested grease in a lubrication system.
Ventmeters have the drawback of being complex and often not available on-site. However, manufacturers of bearings or of automated lubrication systems need to give recommendations regarding grease pumpability and regarding grease rheological properties at the customer site. When using grease with good pumpability, the quantities of lubricants required for the relubrication of bearings may be considerably reduced, which means that the consumption of lubricants will be lowered and the environmental burden will be lessened.
However, following the recommendations requires that simple system and method for checking the grease pumpability are available on-site.